Abstract

The human blastocyst forms 5 days after one of the smallest human cells (the sperm) fertilizes one of the largest human cells (the egg). Depending on the sex-chromosome contribution from the sperm, the resulting embryo will either be female, with two X chromosomes (XX), or male, with an X and a Y chromosome (XY). In early development, one of the major differences between XX female and XY male embryos is the conserved process of X-chromosome inactivation (XCI), which compensates gene expression of the two female X chromosomes to match the dosage of the single X chromosome of males. Most of our understanding of the pre-XCI state and XCI establishment is based on mouse studies, but recent evidence from human pre-implantation embryo research suggests that many of the molecular steps defined in the mouse are not conserved in human. Here, we will discuss recent advances in understanding the control of X-chromosome dosage compensation in early human embryonic development and compare it to that of the mouse.This article is part of the themed issue 'X-chromosome inactivation: a tribute to Mary Lyon'.

X-chromosome dosage compensation in mouse and human. In human pre-implantation development, XIST becomes expressed from all X chromosomes upon zygotic gene activation. As pre-implantation development progresses, XIST expression from both female X chromosomes increases, but remains low in males. The former correlates with dampened gene dosage from both X chromosomes of the blastocyst, equalizing X-linked gene dosage of females to that of males. Upon implantation, all cells undergo random XCI, again resulting in dosage-compensation. In mice, XCI happens in two waves. First, Xist is induced only on the paternally inherited X chromosome (P), causing imprinted XCI in all cells of early pre-implantation embryos (morula). As the blastocyst forms, Xist expression becomes suppressed in the ICM cells (but not in the TE), and the Xi reactivates, leading to increased X-linked gene dosage in females compared to males. As the embryo implants, the maternal or the paternal X chromosome becomes randomly chosen to undergo XCI, similar to humans.

X-chromosome states of human pluripotent stem cells. The X chromosome state of conventional (primed) hESCs differs from the ICM of human pre-implantation blastocysts from which they are derived. Primed hESCs are in a post-XCI state with an XIST-coated Xi. Over time in culture, the Xi loses expression of XIST and partially reactivates, undergoing XCI erosion. Primed hESCs with two active X chromosomes can also be derived from ICM outgrowths (far right), potentially capturing an intermediate state of the X chromosome in the transition to XCI. Differentiation does not change the X-chromosome state of any of these primed hESCs. When hESCs are derived from the blastocyst under naïve culture conditions, or when primed hESCs, regardless of their X state, are converted to naïve pluripotency, the X-chromosome state resembles that of the blastocyst, with two active X chromosomes and XIST expression (on one or both X chromosomes). Like normal development, differentiation of naïve hESCs induces XCI. Similar to primed hESC derivation, an XIST-negative state with two active X chromosomes is an intermediate in the primed to naïve hESC conversion, suggesting stepwise reversal of events. The lncRNA XACT is co-expressed with XIST in naïve pluripotency and might be responsible for inhibiting XIST-mediated silencing. XIST and XACT occupy non-overlapping territories on the active X chromosome (green and purple) in naïve hESCs. XACT is also expressed in primed hESCs both from active and eroding/eroded X chromosomes, and it might be driving erosion by interfering with XIST expression or accumulation. XACT is not expressed in differentiated cells.